The present invention relates to a wireless communication system in which a plurality of radio stations perform communication mutually, such as a wireless LAN (Local Area Network) or PAP (Personal Area Network), a wireless communication apparatus, a wireless communication method, and a computer program therefor. Particularly, this invention relates to a wireless communication system in which each communication station performs random access based on carrier detection on a communication channel by the CSMA (Carrier Sense Multiple Access) method, a wireless communication apparatus, a wireless communication method, and a computer program therefor.
More specifically, this invention relates to a wireless communication system in which communication stations performs random access under the wireless-communications environment where a plurality of transmission rates are available, a wireless communication apparatus, a wireless communication method, and a computer program therefor, more specifically to a wireless communication system in which communication stations performing random access transmit information using a proper transmission rate between them; a wireless communication apparatus; a wireless communication method; and a computer program therefor.
Wireless networks which are alternative for the bothering cable wiring in the existing cable communications are brought to a lot of attention recently. Among standard specifications concerning the wireless network is IEEE (The Institute of Electrical and Electronics Engineers) 802.11 etc.
As a method whereby a communication station transmits information over a channel in a wireless network, there are bandwidth reservation transmission in which a bandwidth is secured in advance for a control station called “access point” or “coordinator,” random access in which a communication station having generated transmission data starts transmission indiscriminately, and the like.
Note here that when a plurality of users (communication stations or terminals) try to make random access on the same communication channel, it is necessary to avoid contention. As a typical communication procedure therefor, the CSMA (Carrier Sense Multiple Access) system is known. The CSMA is a connection system of performing multiple access based on carrier detection. Since it is difficult to receive a signal transmitted by itself in wireless communication, a communication station checks absence of information transmission of another communication apparatus by the CSMA/CA (Collision Avoidance) system rather than the CSMA/CD (Collision Detection) system, and then starts to transmit information of its own, so that the contention of access is avoided.
Moreover, it is known that a problem of a hidden terminal occurs under a communication environment in which each communication station makes random access autonomously. The hidden terminal indicates a communication station that, in the case of performing communication among certain specific communication stations, is available for hearing from one communication station serving as another party of the communication but that is unavailable for hearing from another station. A communication station wishing to start information transmission is unable to negotiate with the hidden terminal, and accordingly there is the possibility that transmission operations may cause collision if only the CSMA/CA system is operated.
As a methodology for solving the hidden terminal problem, there is a system of executing handshake in which a communication station wishing to transmit information sends a transmission request packet prior to transmission start to a communication station acting as a receiving side, and the reception station sends back an acknowledgment notification packet to start the communication. As its representative example, the RTS (Request to Send)/CTS (Clear to Send) procedure can be mentioned. This procedure is also adopted in IEEE802.11.
In this system, the procedure shall be followed that a communication station that is a sender of data sends an RTS packet, a communication station that is a destination of the data sends back a CTS packet, and in response to reception of the CTS packet, the sender starts the data transmission. Then, when a hidden terminal received at least either of the RTS or the CTS, the hidden station sets up an NAV (Network Allocation Vector) only for a transmission stop duration “Duration” of its own station equal to a duration in which data transmission based on the RTS/CTS procedure is expected to be performed, whereby collision maybe avoided. Upon reception of the CTS, the hidden terminal to the transmission station sets up the NAV for a transmission stop duration, so as to avoid collision with a data packet; upon reception of the RTS, the hidden terminal to the reception station stops a transmission duration, so as to avoid collision with an ACK.
The transmission stop duration depends on a value of a transmission data length divided by a transmission rate. Usually, a communication station transmitting a packet is prompted to describe Duration value for which NAV shall be set up in the MAC header. Peripheral stations that are not another party of communication analyze the MAC header of a transmission frame, and sets up the NAV for a duration corresponding to the Duration value, so as to evade a transmission operation.
Here, in the case where a packet is transmitted at a transmission rate that is not supported by the peripheral stations, it is difficult for the peripheral stations to set up the NAV for a proper transmission stop duration, which causes a problem that collision can no longer be avoided. As this solution, it is conceivable to adopt a method in which a communication station in conformity to IEEE802.11n supporting high-speed transmission rate spoofs a Phy header so that a communication station in conformity to IEEE802.11a may set up a transmission stop duration properly. For example, a description of JP-A2004-366912 that has already been transferred to the present applicant discloses a method of allowing the peripheral stations to acquire a correct transmission stop duration by spoofing the transmission data length and the transmission rate in the Phy header of a packet.
When a plurality of transmission rates coexist in a single system, a handshake procedure to check a transmission rate used in communication between communication stations that perform transmission and reception of packets becomes necessary. Since the transmission rate as referred to herein is expressed by a combination of a communication band and a modulation scheme, it is also called MCS (transmission rate) (Modulation Coding Scheme). That the transmitting side requests the receiving side for a transmission rate is called MRQ (MCS (transmission rate) request); that the receiving side feeds back a transmission rate to the transmitting side is called an MFB (MCS (transmission rate) feedback).
It is possible for the transmitting side to determine a transmission rate based on significance of transmit data. For example, in the case of more significant data, a lower transmission rate will be applied, so that a receiving error in the receiving side is avoided. On the other hand, since the receiving side can acquire channel information using a preamble part of a packet, high (i.e., optimum) transmission rate that is allowed depending on ever changing channel situation can be known.
Duration is calculable using a transmission rate that was fixed between the transmitting and the receiving sides. However, if the determination right of a transmission rate was given to either of the transmitting or the receiving communication station, since the Duration value has already been set up using a specified transmission rate, it is difficult for a communication station without the determination right of a transmission rate to revise the transmission rate. Since a peripheral station sets up the NAV with the already received Duration value, if the transmission rate is changed freely, it may change a transmitting duration of a transmission frame and may invite a collision.
In the present IEEE802.11n, because MCS feedback and setting of Duration are closely linked, a transmission station is given a determination right of MCS (transmission rate) and does not prioritize an MCS specified by the receiving side (see Non-patent Document 1). Moreover, because the Duration value will be set up based on a transmission rate specified by the transmitting side, it is difficult for the receiving side to revise this.
The receiving side is allowed to know a transmission rate that meets a channel situation. However, as specified by IEEE802.11TGn, in the case where the transmitting side is intended to have the determination right of a transmission rate, the transmitting side has no choice but to indiscriminately determine a transmission rate because the transmission side is unable to receive a feedback of a reception rate that the receiving side recommends.
[Non-patent Document 1]
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